A review of the literature on the relationship between thyroid disorde
rs and breast cancer does not provide conclusive evidence for the esta
blishment of a causal relationship as breast cancer has been associate
d with hypothyroidism, hyperthyroidism and nontoxic goiter. Most repor
ts on the association of thyroid enlargement and breast cancer have em
anated from areas of endemic iodine deficiency and these reports have
relied on neck palpation. The contribution of the present study is in
the application of the highly sensitive technique of diagnostic ultras
ound to the investigation of subtle changes in thyroid volume and anat
omy in patients with breast cancer. The mean thyroid volume of 20.4+/-
1.0 ml in 184 breast cancer patients was significantly greater than th
at of 12.9+/-1.2 ml in age-matched controls (p<0.01). Also the number
of individual patients with breast cancer having enlarged thyroid glan
ds (73/184; 39.7%) was significantly greater than the corresponding nu
mber (13/150; 8.6%) in the control group (p<0.001). There was a direct
correlation between thyroid enlargement and breast tumour staging. Bo
th mean thyroid volume and % of enlarged thyroids were identical in gr
oups of patients scanned after (Retrospective Group) and before (Prosp
ective Group) various therapies for breast cancer, thus excluding ther
apeutic intervention as a cause for thyroid enlargement. The results i
ndicate a direct relationship between the two disorders perhaps involv
ing a common growth stimulus and emphasise the importance of raising t
he consciousness of the coincidence of both disorders.